The present invention relates generally to screw presses for extracting liquids from semi-solids. More particularly, the present invention is directed to an improved screw press for the mechanical separation of liquids from semi-solid material. The invention is believed to have particular application to the extraction of liquids from manure. However, the screw press of the present invention is believed to also have application to the extraction of liquids from a wide variety of materials, such as the extraction of liquids from fruits or vegetables, the pressing of grapes within the wine industry, and the processing of garbage and trash for reducing the volume thereof.
Presses suitable for the extraction of liquids from semi-solid materials have been known for a very long time, and there are various press structures that have been utilized. The most popular is the screw press. Screw presses include longitudinally extending shafts having enlarged threads. The shaft and screw are encased within a housing. Screw presses work because the rotation of the shaft and threads causes materials to be conveyed through the housing to a restriction. The restriction, also commonly referred to as a strainer, includes an orifice, cone, mesh or screen. Because screens are one of the more popular straining devices and considered most useful for application of the present invention, screw presses will be described herein as including a screen restriction. However, other restrictions may be also utilized in the practice of the present invention.
In operation, the screw press forces the semi-solid material towards the screen causing the material to compress and squeeze liquid through the screen openings for capturing in a large tank or other container. Some screw presses use a variable decreasing thread also referred to as a flight, to compress the material between the threads. Unfortunately, these types of presses are prone to the machinery jamming because too much material can be compressed between the flights restricting rotation of the shaft. Moreover, the material's “head” can be too full of moisture content to act as a plug at the discharge restriction. Variable flight designs also place a lot of radial pressure on the screens which can shorten the screen's life.
Still additional problems have been encountered with cantilevered screw presses. In a cantilevered press, the screw is driven from the drive end but has no support at the discharge end. The discharge end will typically include a closed door immediately past the screen. Material introduced to the feed screw is carried forward and built up in the screen chamber and forced against the screen and closed door. Once the cake is built up, material continues to be fed into the cantilevered presses causing liquids to be squeezed out of the cake and passed through the screen. Unfortunately, the weight of the screw shaft can cause the shaft to bend causing the screw threads to engage the screen and/or interior housing. Moreover, it is extremely difficult to engineer a screw shaft to be perfectly parallel to its annular housing which also causes the screw threads to engage the housing and/or screens. Because the screw threads, housing and screens are typically made of metal, this results in a metal-to-metal contact creating significant noise, wear and friction which hinders rotation of the screw shaft. The cantilevered screw press has another shortcoming in that excessive pressure at the discharge end requires a high rate of shaft revolution which can cause significant wear to the screw press components, or requires a long screw to separate the high pressure end from the low pressure end causing a long and costly design. Moreover, an equilibrium pressure will often occur somewhere along the shaft causing the high pressure end of the shaft to push or pull back towards the inlet and cause the inlet to fill with the semi-solid material. This will inhibit the proper movement of material toward the high pressure side of the press. The material will then just “roll” with the rotation of the screw between the screw threads.
To overcome some of these problems, screw presses will sometimes employ a radial gate valve, also referred to as a “star wheel”, to inhibit back-flow of material in the press. The radial gate valve includes a plurality of teeth which project into the interstitial spaces between the screw's threads which inhibit the back-flow of material. Unfortunately, radial gate valves are expensive to manufacture and often do not impart sufficient force upon the material to completely prevent material back-flow.
Still additional screw press designs limit the amount of pressure at the material head and, consequently, less liquid is removed from the product before the solid material is pressed and discharged. In order to counter these problems, it has been known to use “super chargers” which build pressure within the inlet of the screw press to force material through the screw press to increase head pressure. These super-charged screw presses also reduce the tendency of material to back-flow towards the inlet which can stop all production. Thus, there are significant disadvantages with all prior screw press designs.
Therefore, it is an object of the present invention to provide an improved screw press which increases the liquid extraction from semi-solid materials.
It is still an additional object of the present invention to provide an improved screw press which reduces wear to components structures.
It would be an additional object of the present invention to provide an improved screw press which is inexpensive to manufacture, simple to operate and less prone to breakdowns.
These and other objects, features and advantages of the present invention will be apparent from the following written description which follows.